Basically, it's telling you the hard drive doesn't have an operating system on it.
If this is a second drive you just added, double check the jumpers.  Make sure your
old c drive is set to master, and this new one to slave.  If it's the only drive, and
you've verified it's master, and you know it had an operating system on it from
another system, your problem may be that the 486 BIOS can't recognize the whole
drive.  The fix for that is a BIOS update if the system board can be flashed.  If it
can't be, use a utility like Disk Manager.  DM loads a driver that lets you overcome
the BIOS to see the whole drive.

Doug & Sharon White wrote:

>         I have a strange problem.  I have an old 486-100.  I was changing out the
> hard drives in it and when I went to reboot I got this error message:
>
>         "Invalid system disk. Remove disk and press any key to continue."

                         PCBUILD's List Owner's:
                      Bob Wright<[log in to unmask]>
                       Drew Dunn<[log in to unmask]>